Heat flows away from your body in 4 ways; radiation, conduction, advection by air, and the heat carried in water vapor. Even a loose shawl will reduce radiation on clear night. To stop conduction you need avoid contact with cold objects. To stop heat loss by advection in air you need to stop the movement of air past your skin. Standard hand knit items (http://www.yarnstandards.com/weight.html) are so loosely knit that air goes through them like race cars go around a race track, and every time any cool air goes past your skin, it picks up another bit of heat and carries it off. When a sweater is given substantial ease, then any movement of the fabric with respect to the skin will actually cause the sweater fabric to pump air under the garment increasing ventilation and heat loss. Even a very heavy sweater, if it is loosely knit or it hangs loosely will not be very warm. It will just be heavy, not warm.
Woven materials excel at being tight enough to stop advection by air. However, most woven materials are so thin that heat can be conducted through them. That is, heat is advected or radiated or conducted to one side of the fabric, the heat is conducted through the woven fabric, and then the heat is advected or radiated or conducted off the outside of the fabric. Knit fabrics are thicker and reduce conduction. Knit fabrics are thicker and need to be considered as a three-dimensional volume, with a “skin-side”, an outside, and distance between the two sides. Knit fabrics with Fair Isle or weaving or double knitting or the other double yarn techniques are even thicker and thus even warmer. In the temperature range of liquid water, knit wool fabrics can be thick enough to do an excellent job of blocking heat conduction.
Thus, if you want a warm fabric, it needs to be tight enough to stop air flow and thick enough to reduce conduction. A fabric knit with 2.25 mm needles and worsted weight yarn or 3.25 mm needles and Aran weight yarn will do this - if you full the fabric after knitting.
I do not just "block", I full and block!
On left "as knit", on right after fulling and blocking. Note the "holes" in the as knit fabric. Compared to air molecules, they are HUGE. Air can rush through those holes. After fulling, the channels through the fabric are smaller.
(On the one hand, it is a lot of work to knit that tightly. On the other hand, you will have an extraordinary garment. These are the kind of socks that mothers, wives, and sisters knit for their sons, husbands, and brothers as they prepared to march off to war. These are socks that ooze love when you put them on. No store bought sock is ever as good as a sock that is "knit to fit".)
Cotton, linen, hemp, and nettles will allow water vapor to migrate from the body to the outside of the fabric and then wick the condensed moisture back to the body, where it will again evaporate and move outwards. This process can carry very large amounts of heat away from the body, very, very rapidly. Wool tends to wick less moisture back toward the body. However, water droplets will move through standard hand knit fabrics resulting in the entire fabric tending to be damp if the outside of the fabric has water droplets on it. On the other hand, wool that very tightly knit results in any condensed water droplets remaining on the outer surface of the garment and prevented from passing into the body of the fabric by surface tension. Thus, a damp or wet tightly knit wool garment tends to dry rapidly from the inside out when it is worn, and feels dry and comfortable on the inside, even while the outside is still quite damp or even covered with water droplets. On the hand, a more loosely knit wool sweater does not feel dry until the entire fabric is dry because the fabric is so loose that any droplets of water on the outer surface (including those produced by the condensation of moisture from the body) can move through the body of the fabric, (re)wetting the inside surface of the fabric.
Thus, I can take two sweaters knit from worsted weight wool; one knit at 5 spi and the other knit at 7 spi. The 7 spi sweater will stop the wind, and keep me warm, while the one knit at 5 spi will allow much more air to flow through it and thus not keep me as warm. When it rains or drizzles, the drops will stay on the outside surface of the 7 spi fabric and not penetrate to the inside surface of the fabric, while with a fabric knit at 5 spi the water droplets will move through the body of the fabric. Thus, in wet weather, the 7 spi fabric will feel warm and dry while the looser fabric feels wet.
Ok, you say, “I want a warm gansey, but how will I avoid sweating to death when I stop in to the pub on the way home from _______”. Easy, you knit it snug, but not skin tight. As you warm up in the pub, (or on deck out of the wind) air convection driven by body heat will develop up under the gansey exiting at the neck. It will feel strange at first, but it works. I can wear a gansey skiing, and be the first in the beer line, because everyone else has to stop and take off their layers. After lunch, I can be the first in the lift line because everyone else is putting their layers back on. (The great virtue of the gansey was that a sailor could wear it in the relative warmth of the hold, and yet it would keep him warm when he rushed on deck to shorten sail.) This ventilation can be enhanced by cables and retarded by stitch patterns such as Lizard Lattice. This ventilation is why the traditionally fisherman’s ganseys had the large neck opening. If you want a turtle neck, either use a looser fabric for the turtle neck or expect the pub to be a bit warm. If you knit a skin tight gansey with a turtle neck, you had better plan on wearing it only in cold conditions.
A quick and easy gansey in MacAusland 2-ply fine natural wool knit on 2.35 mm needles with a turtle neck of knit of looser, softer fabric. The garment is very warm and dry under almost any conditions and allows great freedom of motion. By the by, Edie shown here, is a first rate rock climber.
To get the tight knit without ruining my wrists, the above Brown Gansey was knit using a knitting sheath and gansey needles. See previous posts.